I ordered a batch of 10 boards from JLCPCB.

Final design has the following features:

• Z80 CPU - any speed from 4 MHz to 20 MHz should work
• 512KB Static RAM, 55 nS, Zero Wait State data and program memory
• PSoC5LP - Cypress Semiconductor System on a Chip
  • $8 part
  • 32-bit ARM CPU
  • 5V logic levels - all glue logic in PSoC
  • USB interface
  • I2C interface with interrupt input line
  • RTC with 3V 2032 battery
  • Serial connections with handshakes (could use external FTDI or RS-232 transceiver)
  • 8-bit DAC output
  • Emulates Z80 peripheral chips in PSoC firmware
  • Flexible Memory Mapper
    • Maps the 512KB SRAM into the Z80's 64KB space
    • Software-selectable bank sizes from 2K to 32KB
    • Can be configured "on-the-fly" or by compiling PSoC code for particular software
    • Z80 sets the banks with a write in I/O space - compatible with various software memory management schemes
• LED - controlled by PSoC
• Optional I2C Port Expander
  • MCP23017
  • Adds 16 bits of programmable I/O
• Optional oscillator
  • Not required for most applications since PSoC controls all interfaces and has an accurate trimmed internal oscillator
• Connectors
  • I2C
  • USB B
  • Serial TTL-level
• 95x95mm form factor with mounting holes